1. Field of the Invention
Apparatuses consistent with the present invention relate to frequency synthesis, and more particularly, a frequency synthesizer in an ultra wide band (UWB) wireless communication system which transmits and receives data through a multiband.
2. Description of the Prior Art
As access to high capacity multimedia data with high resolution has become commonplace recently, there are increasing demands for a high speed data transmission in a wireless communication system.
For the data transmission, the wireless communication system uses frequencies of a certain band. As the amount of data to be transmitted in the wireless communication system increases, the frequency bandwidth broadens.
Communication data are divided into circuit data and packet data. A frequency bandwidth for the transmission of the circuit data is generally narrow, whereas the transmission of the packet data requires a wider frequency bandwidth. Recently, a concept of a very high speed wireless communication has been suggested for the purpose of real-time transmission of high capacity multimedia data. Consequently, an even wider bandwidth is required. Such a wide frequency bandwidth for the very high speed wireless communication is referred to as an ultra wide band (UWB).
The UWB is a communication with a frequency bandwidth of 3.1 GHz˜10.6 GHz assigned by the U.S. Federal Communications Commission (FCC). The UWB occupies a fractional bandwidth more than 20% and a frequency bandwidth of more than 500 MHz. Since UWB signals use a large frequency bandwidth, a power density value in a frequency domain can be lowered to a small value. Thus, even if the UWB signals superimpose with other communication signals at frequencies, little interference is produced. When they were first proposed and developed, UWB signals obtained a large frequency band based on very short signal pulses. Currently, various UWB techniques have been suggested such as code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM).
Amongst these techniques, a UWB communication system based on a multi-band orthogonal frequency division multiplexing (MB-OFDM) divides a frequency bandwidth into a plurality of sub-bands. The MB-OFDM UWB communication system can transmit more data within the plurality of sub-bands by a unit time. One of the plurality of sub-bands is selected and data is transmitted through the selected sub-band, thereby improving security. In short, the data security can be improved by using the plurality of sub-bands in sequence.
FIG. 1 depicts a frequency bandwidth used in a MB-OFDM UWB communication system. As shown in FIG. 1, the frequency bandwidth is divided into five band groups, and consists of 14 sub-bands. The first band group is mandatory, and the other groups from the second band group to the fifth band group are optional.
In the band formation based on the MB-OFDM of FIG. 1, the first through the fourth band groups each consist of three sub-bands, and the fifth band group consists of two sub-bands.
Center frequencies of the sub-bands in the first band group are 3432 MHz, 3960 MHz and 4488 MHz, respectively. Center frequencies of the sub-bands in the second band group are 5016 MHz, 5544 MHz and 6072 MHz, respectively. Center frequencies of the sub-bands in the third band group are 6600 MHz, 7128 MHz and 7656 MHz. Center frequencies of the sub-bands in the fourth band group are 8184 MHz, 8712 MHz and 9240 MHz. Center frequencies of the sub-bands in the fifth band group are 9768 MHz and 10296 MHz.
It is to be noted that such a band plan base on the MB-OFDM may change in response to further technological development.
As discussed above, a formation of band groups is required to generate signals having center frequencies used in the UWB communication system.
FIG. 2 depicts a conventional frequency synthesizer for generating the center frequencies of the three sub-bands in the first band group.
In FIG. 2, the conventional frequency synthesizer is employed in a direct conversion MB-OFDM UWB system, and includes a local oscillator 10, a phase locked loop (PLL) 20, a ⅛ divider 30, a ½ divider 40, a first single side band (SSB) mixer 50, a selector 60, and a second SSB mixer 70. It should be noted that the term “1/N divider” represents a divider which divides a signal by N.
In the following, descriptions regarding the generation of center frequencies of the three sub-bands in the first band group are provided.
To generate the center frequencies 3432 MHz, 3960 MHz and 4488 MHz of the three sub-bands, the local oscillator 10 is set to a locked oscillation frequency and thus generates a frequency of 4224 MHz.
The PLL 20 stabilizes the frequency of the signal generated at the local oscillator 10.
The ⅛ divider 30 is provided with the frequency signal of 4224 MHz from the local oscillator 10 and generates a modulated frequency signal of 528 MHz by dividing the input signal by eight.
The ½ divider 40 is provided with the signal from the ⅛ divider 30 and generates a modulated frequency signal of 264 MHz by dividing the input signal by two.
The first SSB mixer 50 is provided with the modulated frequency signal of 528 MHz from the ⅛ divider 30 and the modulated frequency signal of 264 MHz from the ½ divider 40, and mixes the provided signals, and thereby generating a modulated frequency signal of 792 MHz.
The selector 60 is provided with the frequency signal of 264 MHz from the ½ divider 40 and the modulated frequency signal of 792 MHz from the first SSB mixer 50, and selects one of the provided signals.
The second SSB mixer 70 is provided with the frequency signal of 4224 MHz from the local oscillator 10 and the signal 264 MHz or 792 MHz selected by the selector 60, and generates desired center frequencies 3432 MHz, 3960 MHz and 4488 MHz of the three sub-bands by mixing the provided signals. In more detail, the second SSB mixer 70 generates the frequency signal of 4488 MHz by adding the frequency signals of 4224 MHz and 264 MHz, the frequency signal of 3960 MHz by subtracting 264 MHz from 4224 MHz, and the frequency signal of 3432 MHz by subtracting 792 MHz from 4224 MHz.
The conventional frequency synthesizer is implemented to generate only the center frequencies of the three sub-bands in the first band group which is currently utilized. This is due to the fact that the semiconductor technology cannot support the sub-bands at a high frequency band. In addition, the stabilized UWB communication without suffering interference in a complicated wireless service environment requires the flexible utilization of frequencies by extending the sub-bands. Ultimately, UWB communications using all of the fourteen sub-bands in the first through fifth band groups should be realized.